The present invention relates to optical devices which, for example, can be used with an optical pickup.
In conventional optical devices such as an optical pickup of an optical disk drive for a compact disc (CD) player or a magneto-optical disk drive, optical assemblies such as a grating or a beam splitter are fabricated individually so that optical devices unavoidably become complicated in arrangement and large in size. Moreover, when such optical assemblies are fabricated on the base of the optical device in a hybrid fashion, optical assemblies should be set up with high alignment accuracy.
A push-pull method or a method using three beams has heretofore been used to detect a tracking signal (tracking servo).
According to the conventional push-pull method, which is one of the tracking servo methods, when a spot of incident light beam is shifted from the track or pit on the disk, +1st order diffraction light and -1st order diffraction light reflected from the disk are given different intensities of light with the result that a far-field pattern becomes asymmetric. Accordingly, signals corresponding to the asymmetry of the far-field pattern are detected by two detectors, for example, and these detected signals are computed by a calculator thereby to detect a displacement of spots of incident light beam.
The tracking servo using the push-pull method needs high alignment accuracy in the fabrication of optical assemblies and has a small margin to cope with a disk skew caused when a lens is shifted in the lateral direction or a disk is warped.
When the lens is shifted, returned light from the disk is shifted in the direction perpendicular to a split line of photo-detecting devices on the photo-detecting surface. There is then the problem that a large offset occurs in the signal. (See, FIGS. 11 and 12 herein.)
FIG. 11 is a diagram showing an optical system for a tracking servo using push-pull method.
In this optical system, split photodiodes PD1 and PD2 with a split line parallel to the tangential direction T of a disk 52, i.e., a recording direction of the disk 52, are disposed at the position distant from a Confocal Plane (CP) of a lens 51 (which is used as a converging means). Then, a differential amplifier 53 computes signals received by these photodiodes PD1 and PD2 in such a way as to compute (PD2-PD1) to generate a tracking error signal TE as a tracking signal.
As shown in FIG. 11, when the lens 51 is shifted in the radial direction R (vertical to the tangential direction T) of the disk 52, beam spots of light beams received by the photodiodes PD1 and PD2 also are shifted in the radial direction R accordingly. As a consequence, even when the proper tracking is made, an equality of tracking error signal TE=0 is not satisfied.
FIG. 12 shows a relationship between detrack (i.e., amount of tracking displacement) and a tracking error signal obtained when the lens 51 is shifted along the radial direction R in the optical system of FIG. 11. FIGS. 12A and 12B shows tracking error signals obtained for two kinds of disks (disks 1 and 2) with different groove shapes, respectively.
Study of FIGS. 12A and 12B reveals that, when the tracking error signal is shifted depending on the lens shift direction, the tracking error signal is not canceled even though the tracking is made properly.
The tracking servo uses the three-beam spot method, as mentioned hereinbefore. In this case, light beam has to pass a diffraction grating so that when recorded information is reproduced, an optical coupling efficiency at which an RF (high-frequency) signal is detected decreases.
When light is returned to the light-emitting portion and returned light is received and detected, light should be split by a beam splitter or a hologram. There is then the disadvantage that an amount of light received by a photo-detecting portion decreases.
In view of the aforesaid disadvantage, in order to reduce the number of optical assemblies and to alleviate alignment accuracy required when optical assemblies are fabricated so that the whole of the optical device can be simplified and miniaturized, there is proposed a CLC (confocal laser coupler) configuration in which a light-emitting portion is disposed at the confocal position of a converging means such as a lens and a photo-detecting portion is formed at the position near the confocal at which the light-emitting portion is disposed.
When a tracking signal is detected based on the three-beam spot method by directly-returned light like the CLC configuration, as shown in the optical system of FIG. 13, outward light and inward light are both transmitted through a grating 55 so that an amount of outward light decreases and that diffraction light beams are caused to interfere with each other on the two photodiodes PD1 and PD2 of the photo-detecting device disposed near a confocal plane CP. Therefore, when a differential amplifier 56 computes the signals detected by the photodiodes PD1 and PD2, it is difficult to obtain a correct result. Thus, the above-mentioned method is not useful in actual practice.
To eliminate the aforementioned offset, there has been proposed an optical device in which split photodiodes are disposed at the confocal position and these split photodiodes perform tracking servo based on the push-pull method (see co-pending patent application Ser. No. 08/603,872 whose title of the invention is "Optical Device"). FIG. 14 shows an example of such optical device.
An optical device 60 shown in FIG. 14 includes a semiconductor substrate 61 on which there are formed a semiconductor laser LD serving as a light-emitting means, a photodiode PD serving as a photo-detecting means, and a reflection plan 62 for reflecting light LF emitted from the semiconductor laser LD in the upper direction of FIG. 14. In FIG. 14, both of the semiconductor laser LD and the photodiode PD are disposed near the confocal position of a converging means (not shown) such as a lens, resulting in the aforementioned CLC configuration being made.
Returned light LR from a disk serving as a recording medium is received and detected by split photodiodes PD (PD1 and PD2) split by a split line of the disk tangential direction T which is parallel to the resonator direction of the semiconductor laser LD.
However, according to this optical device 60, a tracking error signal fluctuates considerably depending on the difference of disk type. In particular, the error signal fluctuates depending on the differences of sizes and shapes of grooves in the disk or in the defocused state.
FIG. 15 shows a relationship between a detrack and a tracking error signal obtained by the optical device 60 shown in FIG. 14 in the case of the defocused state. FIGS. 15A and 15B show tracking error signals obtained for two types of disks (disks 1 and 2) with grooves of different shapes.
Study of FIGS. 15A and 15B reveals that the tracking error signals are inverted in phase depending upon the defocusing direction.
If the phase of the tracking error signal is inverted as described above, then endeavors to effect a proper tracking after the signal processing has been performed become complex, resulting in a complicated detection system. As a consequence, the process for manufacturing optical devices also becomes complicated.